Conventional collectors of solar heat include a dark absorber surface that turns sunlight into heat and a transparent cover for this surface to prevent the heat from escaping. The thermal collection efficiency of such a system is determined by the ratio of the resistance to the flow of heat of the transparent cover to the resistance of the rest of the system. By increasing the thermal resistance of the transparent cover without greatly reducing its light transmission, the efficiency and/or operating temperature of any solar heat collector can be greatly improved. It is estimated that two or three times the energy consumed in heating a well insulated building falls on its surface in the form of sunlight. Thus, an insulation that is transparent when the sun shines would provide most of the heating for a structure over most of the United States if it is coupled with a heat storage system for cloudy weather.
In the past the problems of heat loss through conduction, convection and thermal radiation and control of sunlight have been dealt with independently: convection and conduction losses are reduced using spaces filled with some fine structural material without consideration of admission of sunlight; sunlight has been blocked using coatings or layers on windows to prevent room overheating but without consideration of a technique for admitting more sunlight when it is desired to increase the heat in the room. Thermal radiation loss has been of little concern since in the latter situation heat loss is desirable and in the former it is not considered.
Since the heat losses by conduction, convection and radiation are in parallel to each other and are of similar magnitudes, an insulation is ineffective unless all three are dealt with simultaneously, as heat will leave by the path of least resistance.